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Faculty of Engineering Technology
master thesis
Sustainable Energy Technology
Primary interactions between cellulose and lignin during fast pyrolysis of lignocellulosic biomass
Pedro O. Villavicencio Arzola
Sustainable Process Technology
30/01/2020
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Graduation Committee Prof. dr. Sascha R. A. Kersten Dr. Ir. M. Pilar Ruiz Ramiro Prof. dr. Marteen Arentsen
Master Graduation Assignment for the master’s programme Sustainable Energy Technology
University of Twente Enschede, January 2020
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Abstract
In order to overcome the current necessity for the replacement of fossil resources, fast pyrolysis has
emerged as a promising technology for the efficiently use of biomass as a valuable and sustainable
resource. It is a thermochemical process in which lignocellulosic biomass is converted, in absence of
oxygen at 450-600° C for very short residence times, to obtain mainly bio-oil, gas, and char. Although,
many studies have been developed to understand the behaviour of biomass components during fast
pyrolysis, the interaction between them in the process reactions are known to a lesser extent. The purpose
of this study is to advance on the understanding of the fast pyrolysis of lignocellulose, specifically in
whether there is an interaction between two of the main lignocellulose components, cellulose and lignin.
To do so, a fully operational screen-heater reactor developed in the SPT group has been used to process
samples prepared with blends of cellulose with extracted lignin and cellulose impregnated with potassium
(1, 100, 1000, 10000 ppm) with lignin. Liquid chromatography analytical techniques are used to analyze
the composition of the produced bio-oil. The results indicated that the presence of lignin could have
enhanced the levoglucosan yield in the produced oil, but no effect was observed on the glucose recovery
from the oil samples. In addition, native samples of lignocellulose with different lignin content are tested,
in order to analyze the influence of this component in the pyrolysis product composition. An interesting
trend was found, indicating that both yields of levoglucosan and glucose recovery increased as the lignin
content in the samples also did. These could be attributed to, thermal ejection of cellulose-derived
products from the reaction zone, and also a possible thermal stabilization of anhydrosugars on the
presence of lignin-derived aromatics compounds.
Finally, a state-of-the-art of fast pyrolysis of biomass towards bio-oil applications for heat and power
generation, and as a potential source for platform chemicals are explained; as well as the possible
contributions of this study for its further development are addressed.
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Table of Contents Abstract .................................................................................................................................................... 3
1. Introduction...................................................................................................................................... 7
1.1 Scope ....................................................................................................................................... 8
1.2 Research questions ................................................................................................................. 8
2. Literature review .............................................................................................................................. 9
2.1 Lignocellulosic biomass ........................................................................................................... 9
2.1.1 Cellulose .......................................................................................................................... 9
2.1.2 Hemicellulose ................................................................................................................... 9
2.1.3 Lignin ............................................................................................................................. 10
2.2 Fast pyrolysis of lignocellulosic biomass ................................................................................ 11
2.3 Fast pyrolysis of cellulose ...................................................................................................... 12
2.4 Fast pyrolysis of lignin ............................................................................................................ 13
2.5 Effect of inorganic salts during fast pyrolysis.......................................................................... 13
2.6 Interactions between biomass components during fast pyrolysis ........................................... 14
3. Experimental ................................................................................................................................. 17
3.1 Materials ................................................................................................................................ 17
3.1.1 Feedstocks ..................................................................................................................... 17
3.1.2 Analysis materials .......................................................................................................... 18
3.2 Screen-heater ........................................................................................................................ 18
3.3 Screen-heater set-up ............................................................................................................. 18
3.4 Sample recovery and mass balance ...................................................................................... 20
3.5 Analytical techniques ............................................................................................................. 21
3.5.1 Gas Chromatography (GC) ............................................................................................ 21
3.5.2 Gel Permeation Chromatography (GPC) ........................................................................ 21
3.5.3 High Performance Liquid Chromatography (HPLC) ....................................................... 21
3.5.4 Acid Hydrolysis for quantification of sugars .................................................................... 21
4. Results and discussion .................................................................................................................. 23
4.1 Pure components and model mixtures. .................................................................................. 23
4.1.1 Cellulose-lignin interaction effects on gas, solid and condensed product yields. ........... 23
4.1.2 Cellulose-lignin interaction effects on molecular weight of condensed product. ............. 25
4.1.3 Cellulose-lignin interaction effects on condensed product composition. ......................... 27
4.2 Lignocellulose samples from poplar. ...................................................................................... 29
4.2.1 Cellulose-lignin interaction effects on gas, solid and condensed product yields. ........... 29
4.2.2 Cellulose-lignin interaction effects on molecular weight of condensed product. ............. 32
4.2.3 Cellulose-lignin interaction effects on condensed product composition. ......................... 32
5. Possible contributions of present study towards fast pyrolysis applications improvements ........... 35
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5.1 Applications of fast pyrolysis of biomass ................................................................................ 35
5.2 Reinforcement of fast pyrolysis current behaviour models ..................................................... 36
6. Conclusions ................................................................................................................................... 39
6.1 Recommendations ................................................................................................................. 39
References ............................................................................................................................................. 41
Acknowledgement .................................................................................................................................. 45
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1. Introduction
One of the most important challenges for our society is the replacement of fossil resources as the main
provider for the energetic and chemical industries. This challenge should be addressed simultaneously
through the development of sustainable technologies that would enable the efficient utilization of
renewable resources. The new European energy legislative framework has recently set new targets for
2030, aiming to increase at least 32% share for renewable energy and to reduce at least in 40% the
greenhouse gases emissions from the 1990 levels [1]. In addition, it has been reported that bioenergy is
the largest renewable source in EU and is expected to remain as key component of the energy mix for
the mentioned 2030 targets. Over the past period between 2010-2015, the use of biomass for the
production of energy increased 32%. In the other hand, the use of biomass for the production of raw
materials, as valuable chemicals, also increased by 5.6% under the same period [2].
In order to take advantage of this valuable and sustainable resource, lignocellulosic biomass is nowadays
processed in mainly two ways to efficiently produce energetic appliances. First, the most common and
the earliest way to produce energy for the human being is the direct combustion to produce heat; however,
the low efficiency, ash buildup, and the huge amount of CO2 produced as a by-product represent
enormous disadvantages for this process according to current standards of sustainability [3]. In the other
hand, the conversion of biomass into biofuels and valuable hydrocarbons using thermochemical or
biochemical processes, represent a promising strategy to efficiently use this renewable source.
In thermochemical process of biomass, heat and catalysts are used to transform lignocellulose; in
contrast, biochemical processes utilize enzymes and microorganisms. In comparison with bio-processes,
thermochemical processes occur faster (from seconds to hours). Another advantage of thermochemical
methods, is that they are not feedstock specific and can even process the combinations of feedstocks [4].
Among these methods, gasification and pyrolysis represent two of the most developed processes; where
gasification provides fuel gas, that can be combusted for heat generation, and pyrolysis yields a liquid
fuel that can be used for heating or electricity generation.
Pyrolysis is a thermal decomposition of a substance into smaller components by heating it in the absence
of oxygen. The main benefit of pyrolysis process over gasification is that the reaction conditions favor the
production of vapors that are condensed to obtain an organic liquid, commonly known as bio-oil, which
after post-treatment can be easily stored and transported for a later use as fuel [5]. Among pyrolysis
processes, fast pyrolysis has been highlighted over the last years because the increased efficiency for
the continuous production of bio-oil. The purpose of fast pyrolysis is to prevent further cracking of the
pyrolysis products into non-condensable compounds, therefore primary reactions are dominant [6].
However, it is still very difficult to predict the product composition because it does not only depend on the
feedstock composition, but also on the reaction parameters and reactor configuration (chemical and mass
transfer conditions. Therefore, fast pyrolysis of biomass has not been stablished at industrial scale [7]. In
addition, interactions between the lignocellulose constituents, such cellulose and lignin, during fast
pyrolysis have been studied previously, still without a consensus in the conclusion; making prediction of
biomass pyrolytic behavior even more difficult.
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1.1 Scope
The purpose of this study is to advance on the understanding of fast pyrolysis process of lignocellulose,
specifically in whether there is or not interaction between two of the main lignocellulose components,
cellulose and lignin. To do so, an experimental set-up developed in the Sustainable Process Technology
(SPT) group in the University of Twente has been used, known as the screen-heater; because it combines
very fast heating rates of the sample with also very fast removal of the product from the reaction zone,
and high quenching rate of the condensable product. All these characteristics allow to study the primary
reactions during fast pyrolysis of different samples.
Analyzed samples are prepared with blends of crystalline cellulose with extracted lignin and cellulose
impregnated with potassium salts with lignin. From these experiments, liquid chromatography analytical
techniques are used to analyze the composition of the produced bio-oil. The results are compared with
those obtained for the pyrolysis of pure components, previously developed in this research group using
the same screen-heater set-up under similar conditions.
In addition, native samples of lignocellulose with different lignin content are tested, in order to analyze the
influence of this component in the pyrolysis product composition, using the above mentioned techniques.
Finally, the possible impact of the contribution of this work towards a better understanding of pyrolytic
reactions is addressed.
1.2 Research questions
• Are there any interactions between cellulose and lignin during the primary reactions of fast
pyrolysis of lignocellulose?
• How these interactions between cellulose and lignin possibly occur?
• What is the influence of minerals content in lignocellulosic biomass on the production of
sugars and the molecular weight of the oil produced during fast pyrolysis?
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2. Literature review
2.1 Lignocellulosic biomass
Lignocellulose, also called lignocellulosic biomass, refers to dry plant matter and can be considered the
most abundant and renewable resource of organic carbon on earth. It has attracted considerable attention
in recent years and has been projected to serve as ideal, abundant and carbon-neutral source that will
help mitigate CO2 emissions and therefore the climate change [8]. Nevertheless, in contrast to fossil
resources, lignocellulose is a highly oxygenated, solid, and heterogeneous substance, mainly composed
of three highly functional bio-polymers: cellulose, hemicellulose, and lignin, forming a composite material
which is resistant to bio-chemical conversion, a characteristic commonly known as biomass recalcitrance
[9].
2.1.1 Cellulose
Cellulose is the most abundant biomass component, accounting for approximately 50% by weight of
lignocellulose. It is present in every cell wall of the plants and almost in pure form in the cotton fibers. It is
composed of repeating D-glucose units, a six-carbon ring, also known as pyranose [4]. The three hydroxyl
groups in each ring interact with another one to form intra- and intermolecular hydrogen bonds which give
a crystalline structure to cellulose, and its unique properties of mechanical strength and chemical stability.
It has a linear uniform chemical structure as a linear homopolymer connected by glycosidic linkages, as
shown in Figure 1, in spite of its feedstock origins, but is different in the chain-ends and degree of
polymerization. The degree of polymerization of the native cellulose depends on the source and can reach
more than 5000; depending on the degree of organization of its structure, cellulose is composed of
crystalline (highly ordered) and amorphous (randomly distributed) phases [10].
Figure 1. Building units of cellulose, D-glucose units connected by glycosidic bonds [10].
2.1.2 Hemicellulose
In addition to cellulose, a number of other polysaccharides called hemicellulose are present in
lignocellulosic biomass, accounting for about 15–35% by weight. Hemicellulose surrounds the cellulose
fibers and stands as a connecting link between cellulose and lignin, as shown in Figure 2. Hemicellulose
is mainly composed of pentose and hexose sugar building blocks, such as xylose, arabinose, mannose,
and galactose, which have been used as the structural elements of hemicellulose. While cellulose has a
hydrolysis-resistant crystalline structure, hemicellulose is completely amorphous, with little physical
strength. It is readily hydrolyzed by dilute acids or bases, as well as hemicellulose enzymes [11].
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Figure 2. Representation of hemicellulose structure with a lignin linkage [11].
2.1.3 Lignin
Lignin is a very complex aromatic, three-dimensional and cross-linked polymer that consists of a random
assortment of differently bonded phenylpropane units, as can be seen in Figure 3; these monomers are
categorized as guaiacyl, syringyl and p-hydroxylphenyl units. Lignin is mainly present in the outer layer of
the fibers and is responsible for the structural rigidity, and holding the fibers of polysaccharides together.
Lignin percentage may vary from 23% to 33% in softwoods, 16% to 25% in hardwoods, and for the
lignocellulosic biomass a percentage up to 40% has been reported [12]. Physically, cellulose is coated
with hemicellulose, whose empty spaces are filled up with lignin. Therefore, lignin plays a binding role
between hemicellulose and cellulose within the cell wall.
Figure 3. Complex structure of lignin [13].
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The specific structure of lignin varies depending with feedstock origins and may also have differences
based on the extraction method when isolated from the other components. Monolignol structural units, p-
coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, undergo enzymatic radical coupling
polymerization to produce lignin. It has been identified that peroxidase-mediated dehydrogenation of
monolignol units results in a heterogeneous structure of lignin by the formation of C–C bonds and aryl
ether linkages [13]. These alcohols are referred to as p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S)
units within the lignin structure, as shown in Figure 4. Softwood and hardwood lignin have different
structures, guaiacyl dominant lignin is predominantly found in softwoods, as a polymer of higher fraction
of coniferyl phenylpropane units; on the other hand, guaiacyl-syringyl lignin is typically found in hardwoods
as a co-polymer of both coniferyl and sinapyl phenylpropane units [14].
Figure 4. Monolignol structural blocks and the resulting aromatics present in lignin [13].
2.2 Fast pyrolysis of lignocellulosic biomass
Fast pyrolysis is an advanced technology focused on the production of liquid fuel from biomass, that has
been gaining attention over conventional pyrolysis (slow) because it can have several applications, as is
further discussed in Chapter 5.1. The purpose of fast pyrolysis is to prevent further cracking of the
pyrolysis products into non-condensable compounds, as CO2, CO, etc. The process parameters should
be carefully controlled to give high bio-oil yields, where the most important requirement is a high heat
transfer rate, which can be achieved by finely grounding the feed biomass. The feed is typically heated at
high temperatures (450-600° C) for short residence times (less than 2 s) [8]. Since the reaction takes
place for a very short duration, not only kinetics are involved, but also heat and mass transfer plays a
crucial role in the product outcome. Therefore, the desirable products can be produced by setting process
parameters at optimum conditions.
The fast pyrolysis process typically results in bio-oil, gas, and char yields of 60–70%,13–25%, and 12–
15%, based on dry biomass feed weight [7]. Bio-oil is also referred to as pyrolysis oil, pyrolysis liquid,
pyrolysis tar, bio-crude, wood liquid, wood oil or wood distillate. It is a dark brown organic liquid mixture,
which generally comprises of a great amount of water (15–35 wt%) and hundreds of organic compounds,
such as acids, alcohols, ketones, aldehydes, phenols, ethers, esters, sugars, furans, alkenes, nitrogen
compounds and miscellaneous oxygenates, as well as solid particles [15]. The water content of the bio-
oils depends on the initial moisture content of the feedstock and the water formation. Nevertheless, it is
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still hard to achieve chemically accuracy on identification of some individual components in the bio-oils
due its intrinsic complexity.
The higher heating value (HHV) of the bio-oils typically ranges between 15 and 20 MJ/kg, which is only
40–50% of the conventional oil fuels HHV (42–45 MJ/kg). This is due to the considerable oxygen content
present in bio-oils, which is in the extent of 35-40 % on dry basis weight [7].
2.3 Fast pyrolysis of cellulose
Due to its predominant content in biomass, fast pyrolysis of cellulose has been widely studied regarding
the yield of gas, liquid and solid product and the distribution of specific compounds and their formation
pathways. Most of the studies have focused on the influence of reaction parameters, as well as reactors
configurations (temperature, residence time, heating rate, pressure, and particle size). It has been
demonstrated that bio-oil yield can be increased at pressures close to vacuum, compared to atmospheric
processes [16].
Levoglucosan (1,6-anhydro-β-D-glucopyranose) is considered the most prevalent compound produced
from fast pyrolysis of cellulose. The primary reaction pathways for cellulose pyrolysis proposed involve
the formation of oxygenated compounds; from this it has been established that the cleavage of the
glucosidic bonds initiates the formation of levoglucosan (LG) through the intramolecular rearrangement
of the monomer units [17]. Regarding yields of LG, several numbers have been reported under fast
pyrolysis conditions (from 20 to 60%), depending on different parameters, as the reaction temperature.
To have an idea of this variation, the LG yields (DP1, in Figure 5) obtained at different temperatures from
fast pyrolysis of cellulose are shown, as well as the yields of the sum of others anhydrosugars detected
(DP>2, Figure 5) [18].
Figure 5. Yields obtained from fast pyrolysis of cellulose, DP1 refers to LG and DP>2 is refers to the sum of higher anhydrosugars detected [18].
In addition, the degree of polymerization (DP) of the cellulose also has an important impact on its pyrolysis
behavior. The chemical formula of cellulose is often denoted as (C6H10O5)n, where n refers to the DP. The
average DP for cellulose from woody biomass is at least 9000 and possibly as high as 15,000 [15]. It has
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been confirmed that the formation of LG and other pyrans was directly correlated with the DP of a sample,
and a higher DP favored the formation of pyrans [19].
Apart from LG, the presence of hydroxyacetaldehyde and acetol have also been detected in the
oxygenated compounds from cellulose pyrolysis, with reported yields from 3% to 19% and from 0.8% to
6% respectively, presenting the same trend with the increased temperature (450 to 930° C) as with LG
[16].
2.4 Fast pyrolysis of lignin
Lignin is the most thermally stable component in lignocellulose, therefore is degraded in a broad range of
temperature, from 200° to 500° C or even higher. The fast pyrolysis of isolated lignin, under already
stablished systems, has produced high yields of char (from 30–50%) and low yields of bio-oil (around
30%) mostly composed by a great number of aromatic compounds (phenolics and hydrocarbons). As well
as with cellulose, processing at vacuum close pressures relatively increases the bio-oil yield [16].
Due to the high volatility of the mentioned compounds, it has been very difficult to accurately detect all,
as well as their composition. Nevertheless, an analytical technique, pyrolysis-gas chromatography-mass
spectrometry (Py-GC–MS) has been applied to examine the oil product composition from fast pyrolysis of
pine (softwood) lignin, where more than 50 phenolic compounds were found in the bio-oil, including
guaiacol, 4-methylguaiacol, 4-vinylguaiacol and vanillin. On the other hand, more syringol-type
compounds were detected from the pyrolysis of oak (hardwood) lignin. Therefore, the composition of bio-
oil produced from lignin completely depends on its nature, due to the different chemical structures as
explained in Chapter 2.1.3.
Another important characteristic that has been considered for the characterization of lignin pyrolysis is the
molecular weight. It has been demonstrated that the temperature has no influence in the number average
molecular weight (Mn) of the produced bio-oil. In contrast, pressure at which pyrolysis is carried out, does
have an effect in the Mn of the bio-oil. At vacuum pressure, higher Mn has been obtained compared to the
one obtained at atmospheric conditions [20]. Moreover, the molecular weight distribution has been
considered as one of the most important characteristic of the lignin due to it has a direct influence in the
product distribution of the obtained bio-oil, this could have been explained because heavier molecules
polymerize faster than lighter ones.
2.5 Effect of inorganic salts during fast pyrolysis
Biomass itself contains inorganic minerals, such as K, Ca, Na, and Mg. Depending on the type of biomass
and harvesting method, the total content of these species ranges from 0.5% to 15% of the biomass
composition. Alkali and alkaline earth metals (AAEMs), such as K, Na, Mg and Ca, are the most abundant
inorganic ions in biomass and, despite of their very low concentration, have been demonstrated to
catalyze several pyrolysis reactions, completely changing the pyrolysis product distribution, from yields of
solids, condensable and non-condensable products, to the chemical composition of the resulting bio-oil
[21]. Inorganic salt concentrations as low as 50 ppm have been found sufficient to dramatically change
the resulting pyrolysis product selectivity [8].
These salts, or ashes, catalyze the primary reactions during pyrolysis process leading to the formation of
lower molecular weight species (especially formic acid, glycolaldehyde and acetol) from cellulose;
therefore, lower LG yields are obtained. It has been reported a trend of activity according to LG yields, for
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the following cations this trend has been observed: Na+ > K+ > Ca2+ > Mg2+, where for Na+ produces the
highest LG yield and for Mg2+ is the lowest [22]. In contrast, for several anions the following trend on LG
was observed: Cl- > NO- > OH- > CO32- > PO4
3- [23].
A similar investigation has recently been developed in our work group (SPT), where the effects of
potassium concentrations of 1, 100, 1000 and 10,000 ppm, during the primary products of cellulose fast
pyrolysis in vacuum and at atmospheric pressures has been analyzed [24]. It has been highlighted that
potassium salts are catalytically active during the primary reactions of pure cellulose fast pyrolysis. This
catalytic activity leads to a significantly lower yield of condensed product, as well as lower yields of
anhydrosugars in the bio-oil. In contrast, the production of non-condensable gas increased, even at low
potassium content. Another interesting finding is that the anions that have been used, follow a particular
order of activity; regarding the yield of LG: Cl- > OH- > CO32- [24].
In addition, another important effect of inorganic salts that has been noticed during the fast pyrolysis of
cellulose is the ability to catalyze dehydration and ring fragmentation reactions of the anhydrosugar
components. Levoglucosenone, which is formed from LG when it loses two molecules of water, has been
used to identify dehydration reactions. On the other hand, acetol, which is the simplest hydroxy ketone
structure, has been used to identify fragmentation reactions during fast pyrolysis [24]. In Figure 6, the
levoglucosenone production as a function of potassium concentration during fast pyrolysis of cellulose is
presented, where can be observed that with increase potassium concentration up to 1000 ppm leads to
an increase in the yield of levoglucosenone increases from 0.25 wt% to 1 wt%.
Figure 6. Levoglucosenone production as a function of potassium concentration during fast pyrolysis of cellulose [24].
2.6 Interactions between biomass components during fast pyrolysis
During the past decade, the fast pyrolysis of cellulose and lignin as separate components has been widely
studied. Unfortunately, less information is available regarding possible interactions between the different
biomass building blocks. It is interesting to note that, even investigations about interactions between three
main components have been developed, there is no definite conclusion on whether there is interaction
and how it actually works. An example of this is shown in Table 1 and 2, where several studies that have
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been done are listed according to its conclusion of whether there is or not interaction among the biomass
components.
Table 1. Different studies developed with a conclusion of no interaction among biomass components during fast pyrolysis.
Authors Reactor / Conditions Conclusion
Yoon et al. [25] TG-GC / Air/steam atmosphere, 10°
C/min, 140°-900° C.
No interaction on product yields and carbon
conversions.
Skreiberg et al. [26] TG and macro-TG / Air atmosphere,
5°, 20°, and 100° C/min, 60°-900° C.
No interactions on product composition.
Qu et al. [27] Tube furnace / 350°-600° C No interactions on product yields.
Wang et al. [28] TG / Hydrogen/syngas,10°,15°,20°
C/min, 300° - 600° C
No interactions on gas yields and gas
composition.
Yang et al. [29] TG / Nitrogen atmosphere, 10° C/min,
105°-900° C
Negligible interaction, on product yields (oil,
gas, solid).
Table 2. Different studies developed with a conclusion of interaction among biomass components during fast pyrolysis.
Authors Reactor / Conditions Conclusion
Wang et al. [30] TG-FTIR / Nitrogen atmosphere, 20°
C/min, 300°-800° C
Interaction between cellulose and
hemicellulose inhibits the formation of LG.
Hosoya et al. [31] Ampoule reactor / Nitrogen
atmosphere, 600° C, 40-80 s
residence time
Cellulose – lignin interactions, the cellulose-
derived volatiles act as H-donors while the
lignin-derived volatiles act as H-acceptors.
J.Hilbers et al. [32] TGA / N2 atmosphere, 10° and 50°
C/min
Py-GC-MS / 350° C and 500° C
Cellulose – lignin interactions, lignin
enhanced LG yields, but decreased
dehydration products.
Zhang et al. [33] Py-GC-MS / Helium atmosphere,
500° C
Cellulose – lignin interactions, increased
yields of low MW and furans, but decreased
yield of LG.
Wu et al. [34] Py-GC-MS / 500°, 600°, and 700° C),
mixing mass ratios 1:1, and 2.1:1
Cellulose – lignin interactions, promote low
MW products, while inhibited LG formation.
Yu et al. [35] Wire mesh reactor / N2 atmosphere
50° C to 900° C, heating rate of 30°
C/min
Interactions were observed between
cellulose and the other two components,
while comparing yields of gas, char and oil.
From the lists above, it is remarkable how different conclusions have been reported, even when similar
reactors and conditions are used. One of the reasons for this discrepancy could be that biomass pyrolysis
models are based on the behavior of the three main components, where the effects of interaction on
thermogravimetric (TG) reactors cannot be easily noticed, so people tend to neglect the interactions
between the three main components [34]. Another reason is that usually, the yields of non-condensable
gas, bio-oil and char are always similar to those when pure components are reacted, because there are
no change of H:C:O ratios in biomass, therefore it is really difficult to come to the conclusion with
interaction [33]. However, in order to produce liquid fuels and valuable platform chemicals, detailed
information about the compounds obtained in bio-oil and a complete biomass decomposition mechanism
are needed.
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As mentioned in Table 2, fast pyrolysis has also been studied using Pyrolysis-Gas Chromatography/Mass
Spectroscopy (Py–GC/MS), where crystalline cellulose (Avicel), amorphous cellulose, organosolv lignin,
and their blends containing 20, 50, and 80 wt.% of lignin were used; it was found that the presence of
lignin enhanced the yield of LG, but decreased the yield of some of their dehydration products
(levoglucosenone) [32]. In contrast, cellulose-lignin interactions were also investigated in a Py-GC-MS
with different temperatures (500°, 600°, and 700° C), mixing ratios (mass ratio 1:1, and 2:1), and mixing
methods (physical mixture and native mixture); and it has been found that cellulose-lignin copyrolysis
could promote low weight molecular products (esters, aldehydes, ketones, and cyclic ketones) from
cellulose and lignin-derived products (phenols, guaiacols, and syringols), while inhibiting formation of
anhydrosugars, especially the formation of LG [34]. Therefore, it is necessary to gain more information
about biomass components during fast pyrolysis process in order to better support the models that have
been developed until today.
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3. Experimental
3.1 Materials
3.1.1 Feedstocks
Cellulose Avicel PH101 from Sigma-Aldrich (particle size ~50 μm, 60.5% crystallinity, ash content 0.005
wt%, AAEM content 1 ppm, degree of polymerization specified < 350, average 220) was used as a
representative of cellulose for the blends prepared. Spruce organosolv lignin provided by the Energy
Research Centre of the Netherlands (ECN), of whose isolation process has already been described
elsewhere [36], was also used as lignin representative to prepare the blends.
In addition, cellulose samples impregnated with potassium (100 ppm, 1000 ppm, and 10000 ppm),
previously developed for a study on this research group (SPT) were also used, where potassium
carbonate (K2CO3, Sigma Aldrich, purity > 99%) was used as source of K for 100 ppm and 10000 ppm
samples, and potassium chloride (KCl, Sigma Aldrich, purity > 99%) for 1000 ppm sample. The
impregnation procedure was done dissolving the respective amount of K2CO3 and KCl in Milli-Q water to
then mixing it with pure avicel. The mixture was thoroughly mixed (T = 20° C) in a round bottom flask for
1 h. After mixing, the Milli-Q water was removed using a rotary evaporator (Büchi Rotavapour R-200, T =
65° C, P = 100 mbar, for 1 h). Finally, impregnated cellulose samples were dried for 24 h using a vacuum
oven (Heraeus FVT420, T = 20° C, P = 1 mbar) [24].
From these materials, four blends were created using a physical mixture only. Mixture method was not
important on these experiments due to the characteristics of the screen-heater, that will be deeper
explained. The composition of the blends was kept 70 wt% of cellulose and 30 wt% of lignin under ash-
free basis, as described in Table 3.
Table 3. Composition of the blends used as feedstock, ash-free basis.
Blend Cellulose (% wt, ash free) Lignin (% wt, ash free) Potassium (ppm)
A / L 70 30 1
A / L 100 70 30 100
A / L 1000 70 30 1000
A / L 10000 70 30 10000
Apart from these blends, four more samples were used as feedstock for the experiments, which were
kindly provided by the PhD candidate Thimo te Molder, also from STP group. These samples, obtained
from poplar wood, contain different lignin content, as can be seen in Table 4.
Table 4. Poplar samples content.
Sample Lignin content
(% wt)
Ash content
Poplar 25 High
Poplar AL 25 Low
PT19 11.5 Low
PT17 3.6 Low
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3.1.2 Analysis materials
The condensed product was recovered from the reactor by rinsing it with tetrahydrofuran (THF, Merck LC
LiChrosolv, Purity≥99.8%) or with Milli-Q water, depending whether the GPC or HPLC, respectively, was
used for analysis. A dissolution of sulfuric acid at 72% v/v, was prepared in the SPT group with sulfuric
acid (Sigma Aldrich, purity 99.99%) and was used for the hydrolysis of the condensed product to analyze
its sugar content; and barium carbonate (Sigma Aldrich, purity >99%) was used to neutralize the
hydrolyzed samples before HPLC analysis. Levoglucosan (1,6-Anhydro-b-D-glucopyranose, Carbosynth
purity >98%) and glucose (Sigma-Aldrich purity 99.5%) were used as standards for calibration of the
HPLC.
3.2 Screen-heater
The screens used for the reactor were cut 2.5 cm x 5 cm, from a large stainless steel wiremesh (Dinxperlo,
Wire Weaving Co. Ltd, mesh 200 wire thickness 0.06 mm * 0.06 mm, twilled weave, AISI 316). The
screens were cleaned with Milli-Q water followed by acetone (Sigma Aldrich, purity >99.5%) before use
for each experiment.
Approximately, 40 to 60 mg of feedstock for each experiment was evenly distributed over one screen, by
using a sieve in order to facilitate the process. A little space of 0.5 cm of the screen was kept free of
feedstock in order to be able to clamp the screen between the electrodes. A second screen was pressed
on top of it with a hydraulic press (Rodac RQPPS30 30t) at 250 kg/cm2. The exact amount of feedstock
between the screens was determined by weighing the pressed screens, including the feedstock, on an
analytical balance (Mettler Toledo, max 220 g, readability 0.01 mg) and subtracting the weight of the initial
screens. In Figure 7 is shown the front view of the screens with the feedstock sample between.
Figure 7. Screens coupled together with sample in between after press.
3.3 Screen-heater set-up
Before each experiment the screens & feedstocks (Figure 8.2), copper clamps (Figure 8.5), vessel (Figure
8.1) and Teflon tape (two pieces of 70 cm long) (Figure 8.6) were weighed. The screen-heater reactor
consists of a glass vessel (Duran 250 mL, round bottom, D=5 cm). In this glass vessel, the pressed
screens with between the feedstock in between (Figure 8.2) were clamped between two electrodes
(Figure 8.5). Teflon tape was wrapped around the electrodes to be able to quantify the condensed
19
products on this part. After installing, a vacuum was created of 0.05 mbar using a vane vacuum pump
(10, Edwards E2M-1.5). The reactor was filled with nitrogen gas at least three times to remove as much
as possible any remaining oxygen inside. After this procedure again a vacuum was created of 0.05 mbar.
A liquid nitrogen bath (Figure 8.4) was placed around the vessel to cool the vessel wall to approximately
-100 °C.
Figure 8. Schematic representation of the screen-heater reactor.
All the experiments made in this study were performed at 0.05 mbar. After the experiment the valve was
opened and the reactor was warmed to ambient temperature. In addition, a 10 mL syringe was installed
in all experiments. This syringe was used to collect a gas sample (Figure 8.8) after each experiment.
The pressure was precisely monitored during the experiment using an accurate pressure gauge (Heise
DXD3765, Figure 8.8). A pyrometer (Figure 8.12) was used to monitor the temperature. To prevent
disturbance in measuring the thermal radiation emitted by the screens while the liquid nitrogen is added
during the experiment, a glass tube (Figure 8.13) with a silicone sealing (Figure 8.14) was placed in the
liquid nitrogen bath. During the experiment an electrical current was passed through the wires, which
served as an electrical resistance heater. This method supplies the heat required for heating the sample
and maintaining it at the set temperature, the final screen temperature (TFS), for a specified time called
the holding time of 5 seconds.
It has been found in previous works with the screen heater [18], [24], [20], for the pyrolysis of cellulose,that
upward of 530 °C the condensable product yield became constant, and optimal yield of condensed
20
product can be achieved with less than 1 wt% of solid residue, and gas yield; therefore, all experiments
in this work were performed at a TFS of 530 °C.
The required power was supplied by two sets of two batteries connected in series. For the heating period
these were two Varta Silver Dynamic batteries (12 V/100 Ah, 830 A) and for the supply of heat during the
reaction two Varta Pro Motive batteries (12 V/225 Ah/1150 A) were used. The temperatures and pressures
were recorded using a DAQ card (NI PCI-6281). The data was processed in a Labview program, which
regulated the screen temperature via a PID control system.
After everything was in position, the pyrolysis run was started following a pre-programmed procedure
using the Labview program (with the set temperature, holding time and PID value). After the run, the setup
was removed from the liquid nitrogen bath and consequently warmed till ambient temperature.
3.4 Sample recovery and mass balance
In order to recover the pyrolysis products after each experiment and to obtain the yields of each product
(non-condensable, condensable and solid) and the mass balance closure, to know the reproducibility of
the experiments; the following procedure was applied. After the experiment was performed and the reactor
reached ambient temperature, it was filled with dry nitrogen gas until the pressure in the reactor reached
approximately 950 mbar. A gas sample was taken from the reactor using a 10 ml syringe. Gas samples
were analyzed using gas chromatography (GC). The gas yield was calculated as shown in Equation 1:
𝑌𝑔𝑎𝑠 = ∑
𝑃𝑠𝑎𝑚𝑝𝑙𝑒 𝑉𝑜𝑙%𝑖 𝑉𝑣𝑒𝑠𝑠𝑒𝑙
𝑅𝑇𝑎𝑚𝑏𝑖𝑒𝑛𝑡𝑀𝑊𝑖
𝑀𝑓𝑒𝑒𝑑𝑠𝑡𝑜𝑐𝑘
𝑛𝑖=1 (Eq. 1)
where, Mfeedstock is the mass of dry cellulose feedstock. Where, i is the number of components
detected in the gas sample, produced by pyrolysis reactions (CO, CO2, etc.); Psample is the pressure at
which sample is taken; Vol%i is the volume percentage of each component i in the gas sample, determined
by GC; Vvessel is the volume of vessel; MWi is the molecular weight of component i; Tambient was the
temperature at which sample was taken; R is the universal gas constant.
As mentioned before, to obtain the yield of the solid residues produced, the weight of the screens was
measured with and without feedstock before the start of the experiment. After the experiment the screens
are weighed again. However, the solid residue was not corrected for salts. The yield of solid residue can
be calculated as shown in Equation 2:
𝑌𝑠𝑜𝑙𝑖𝑑 𝑟𝑒𝑠𝑖𝑑𝑢𝑒 =𝑀𝑠𝑐𝑟𝑒𝑒𝑛𝑠+𝑟𝑒𝑠𝑖𝑑𝑢𝑒− 𝑀𝑠𝑐𝑟𝑒𝑒𝑛𝑠
𝑀𝑓𝑒𝑒𝑑𝑠𝑡𝑜𝑐𝑘 (Eq. 2)
where, Mscreens + residue, Mscreens are the mass of the screens with and without the solid residue,
respectively. After every experiment, the glass vessel (Figure 8, 1) was removed and immediately
weighed. After that, the glass vessel was sealed with parafilm to prevent evaporation of volatile
compounds. The tape (Figure 8, 6) was removed from the electrodes, clamps and screws (Figure 8, 5)
and weighed together. The yield of condensed product was calculated by the following Equation 3:
𝑌𝑐𝑜𝑛𝑑𝑒𝑛𝑠𝑒𝑑 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 =(𝑀𝑣𝑒𝑠𝑠𝑒𝑙+𝐶𝑃−𝑀𝑣𝑒𝑠𝑠𝑒𝑙)+(𝑀𝑇𝐶𝑆+𝐶𝑃−𝑀𝑇𝐶𝑆)
𝑀𝑓𝑒𝑒𝑑𝑠𝑡𝑜𝑐𝑘 (Eq. 3)
21
where, Mvessel + CP, Mvessel are the mass of the vessel with and without the condensed product,
respectively. Similarly, MTCS + CP, MTCS are the mass of the tape, clamps, and screws with and without
condensed product, respectively. The mass balance closure is calculated as shown in Equation 4:
𝐵𝑎𝑙𝑎𝑛𝑐𝑒 𝑐𝑙𝑜𝑠𝑢𝑟𝑒 (%) = (𝑌𝑔𝑎𝑠 + 𝑌𝑠𝑜𝑙𝑖𝑑 𝑟𝑒𝑠𝑖𝑑𝑢𝑒 + 𝑌𝑐𝑜𝑛𝑑𝑒𝑛𝑠𝑒𝑑 𝑝𝑟𝑜𝑑𝑢𝑐𝑡) ∗ 100 (Eq. 4)
3.5 Analytical techniques
3.5.1 Gas Chromatography (GC)
In order to know the composition of the non-condensable product of the pyrolysis reaction, and to be able
to compute the gas yield produced; a micro-GC (Varian MicroGC CP4900, 2 analytical columns, 10 m
Molsieve 5A, 10 m PPQ, Carrier gas: Helium) calibrated for N2, O2, CO, CO2, CH4, C2H6, C2H4, C3H6,
C3H8, was used. The gas samples taken after the screen-heater pyrolysis experiments were analyzed
twice to ensure reproducibility. It is important to mention that, for all the experiments done in this work,
only in a few CH4 was detected, while heavier gases were not detected.
3.5.2 Gel Permeation Chromatography (GPC)
The molecular weight distribution (MWD) of the condensable product obtained was determined by using
GPC (Agilent Technologies 1200) equipped with three columns (7.5mm×300 mm, particle size 3 μm)
placed in series packed with highly cross-linked polystyrene-divinylbenzene copolymer gel (Varian
PLgelMIXED E). The column temperature was maintained at 40° C during analysis. Samples were
dissolved in tetrahydrofuran (THF) in a 1:100 mass ratio and were filtered through a 0.45 μm Whatman
RC Agilent filter. THF was used a mobile phase at a flowrate of 1 mL min. A variable wavelength detector
(VWD, 254 nm), already available in the equipment, was used for the computation. The calibration line
was made with ten polystyrene standards with molecular weights ranging from 162 Da to 29510 Da.
3.5.3 High Performance Liquid Chromatography (HPLC)
Levoglucosan and glucose in the condensed product were analyzed using HPLC (Agilent 1200 series,
column: Hi-Plex-Pb maintained at 70° C, mobile phase: Milli-Q water, 0.6 mL/min). All samples were
measured its pH and neutralized with barium carbonate if needed, then, were filtered (Whatman RC
Agilent 0.2 mm filter) prior to analysis. The quantification of LG and glucose in the condensed product
was done by using a four-point calibration.
3.5.4 Acid Hydrolysis for quantification of sugars
The total amount of hydrolysable anhydro-sugars (including oligomers) were determined, after hydrolysis,
as glucose. The NREL LAP method ‘‘Determination of sugars, byproducts, and degradation products in
liquid fraction process samples” was used [37]. To do this, 20 mg–50 mg of condensed product was
dissolved in Milli-Q water, while rinsing the vessel after the experiment was carried out. The diluted sample
was then added 350 μL of concentrated H2SO4 (at 72% v/v) to obtain a final concentration of 3.5 vol.%.
Finally, the sample was heated in an autoclave at 120° C for 1 h. After hydrolysis the condensed product
was neutralized by adding BaCO3. The condensed product was then filtered to remove the formed
precipitates. The glucose concentration was determined using HPLC. Glucose is a representative of all
hydrolysable sugars including levoglucosan, cellobiosan, cellotetrasan, etc. The yield of hydrolysable
anhydrosugars is therefore expressed as the ratio of carbon in glucose to carbon in cellulose. This method
(NREL LAP) needs a correction for the recovery products, because even when using pure glucose, it is
22
not possible to recover it 100%; therefore, the data obtained in previous work in STP [18], was used to
correct the glucose recovery from levoglucosan at different known concentrations, as described
elsewhere [18].
23
4. Results and discussion
4.1 Pure components and model mixtures.
4.1.1 Cellulose-lignin interaction effects on gas, solid and condensed product yields.
Firstly, is was analyzed whether exist cellulose-lignin interactions have an effect on the products yields of
primary reactions during fast pyrolysis of blends listed on Table 3, where concentration of potassium (K+)
in cellulose was varied between 1 and 10,000 ppm; as well as the fast pyrolysis of pure components in
order to compare the obtained results. The cellulose impregnated samples were predetermined in a
previous work done in SPT group [24].
The non-condensable gas yields, at 0.05 mbar, and 530° C, is shown in Figure 9. It can be seen that the
highest non-condensable gas yield is produced at the blend with highest potassium concentration (0.033
g/g sample). This behavior is similar at which has been reported from previous work, in the screen-heater
with slightly different conditions (5 mbar, 530° C) [24]. Therefore, even if the vapor phase reactions are
suppressed in the screen-heater, the primary reactions catalyzed by potassium salts are the main
responsible for the non-condensable gas production.
In addition, it is important to mention that gasses like CH4 or higher, typically found in gas composition of
pyrolysis processes [7], were not detected in any of the experiments. However, few experiments shew a
small concentration of H2 (less than 0.01% respect to the total gas production). It was seen that the
individual yield of CO2 is dominant for all blends experiments; being the highest for the blend with highest
K+ concentration. Once again, this behavior was similar to that already reported for only cellulose
impregnated experiments [24].
Figure 9. Gas yield obtained of avicel, lignin and their blends.
On the other hand, the solid residue yields are presented in Figure 10. It can be seen that, for pure avicel
experiments, the lowest yield of solid is obtained (0.029 g/g sample); in contrast, for pure lignin
experiments the highest yield was produced (0.166 g/g sample). For the blends, a general trend can be
observed: when the K+ concentration increases the solid residue yield also increases; trend which was
also observed in previous work [24] for cellulose pyrolysis. The solid residue yield to 0.059 (g/g sample)
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
Gas
yie
ld (
g/g
sam
ple)
Non-condesable gas yield
Avicel Lignin A/L A/L 100 A/L 1000 A/L 10000
24
when avicel and lignin are mixed, compared to 0.029 (g/g sample) when pure avicel is used. From these,
it can be said that the presence of lignin increased the yield of solid residue, when the concentration of
the salts is low.
Figure 10. Solid residue obtained from avicel, lignin and blends experiments.
To analyze the most important product for a pyrolysis process, Figure 11 shows the condensed product
yield obtained in the experiments with pure components and blends. It can be seen that, as an effect of
the presence of lignin, the condensed product yield decreases from 0.91 to 0.85 (g/g sample) when avicel
and avicel/lignin are used as feedstock respectively.
In addition, a clear trend can be observed when the concentration of salts increased, decreasing the
production of condensable yield, till 0.58 g/g sample when the concentration of K+ was 10000 ppm. The
pyrolysis at vacuum pressure (0.05 mbar) implies high product escape rates of reacting particle from the
reaction zone; however, even in this short time, potassium causes the oil yield to decrease as the
concentration of salts increases. This trend was also presented in previous work [24]; therefore, it can be
said that the presence of lignin does not have a significant influence in the production of pyrolysis oil, for
the conditions hereby used.
0.000
0.020
0.040
0.060
0.080
0.100
0.120
0.140
0.160
0.180
0.200
Sol
id y
ield
(g/
g sa
mpl
e)
Solid residue yield
Avicel Lignin A/L A/L 100 A/L 1000 A/L 10000
25
Figure 11. Condensable product yield obtained from avicel, lignin and blends experiments.
The obtained mass balance closure of all experiments was between 78% and 95%, being for avicel the
highest, and for the blend with the highest salt content, the lowest; as was expected. It has been already
explained that the reason for a not complete mass balance closure is that water and some of the light
oxygenates lost during dismantling of the set-up [18]. The number of experiments in the screen-heater
performed, for each feedstock used, was at least 6, except for the A/L 10000 blend (only 4). As shown in
Figure 12, the reproducibility of the experiments was satisfactory.
Figure 12. Mass balance closure obtained from avicel, lignin, and blends experiments.
4.1.2 Cellulose-lignin interaction effects on molecular weight of condensed product.
The molecular weight distribution (MWD) of bio-oils represents an important characteristic towards its
possible upgrading for use as fuel or as raw chemical source [13]. Therefore, the MWD, as well as the
molecular numbers, mass average molecular weight (MW) and number average molecular weight (Mn)
were also analyzed in this work, in order to see any potential effect of cellulose-lignin interactions during
fast pyrolysis reactions.
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
Con
dens
able
pro
duct
yie
ld (
g/g
sam
ple)
Condesable product yield
Avicel Lignin A/L A/L 100 A/L 1000 A/L 10000
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Clo
sure
(%
)
Mass balance closure (%)
Avicel Lignin A/L A/L 100 A/L 1000 A/L 10000
26
Figure 13 shows the MWD of the condensed product from avicel/lignin blends used in this study, as well
as the MWD of the condensed oil produced from the pyrolysis of lignin only. If we take the last one as
basis to compare the behavior of the MWD presented, we cannot observe a possible effect of the
cellulose-lignin interactions, for the conditions used. In contrast, the effect of the K+ concentration in the
blends can be noticed for molar mass less than 200 g/mol; and also for molar mass above 300 g/mol, it
is here where a trend can be observed: for lignin only and A/L samples the behavior of MWD is similar, it
decreases when the concentration of potassium increases.
Figure 13. Molecular weight distribution (MWD) of condensed product from blends experiments.
This trend can also be observed when the molecular numbers (Mn and Mw) and the polydispersities
(Mw/Mn) are compared in Figure 14. Firstly, it can be seen that the molecular numbers of the condensed
product decrease as the concentration of potassium increases, being for the lignin only the highest (Mw
= 837 g/mol) and for the blend A/L 10000 the lowest (Mw = 517 g/mol). This could mean that oligomers
generated from pyrolysis decreased as the concentration of potassium potentially increased . In contrast,
polydispersity value (Mw/Mn), which is also a parameter for a material to be considered with potential for
use as fuel [14], was observed to increase as the potassium concentration also increased. These values
were 2.82 for the condensed product of lignin only, and 3.68 for the bio-oil obtained with the A/L 10000
sample.
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
100.00 1,000.00 10,000.00
VW
D s
igna
l/MM
D
Molar mass (g/mol)
Lignin
A-L
A-L 100
A-L 1000
A-L 10000
27
Figure 14. Molecular weight numbers and polydispersity of condensed product from blends experiments.
In order to be able to observe a clearer trend of the MWD behaviour, these values were discretized into
50 segments of 200 g/mol, between 1 and 10000 g/mol and divided by the total to obtain the MWD in
terms of mass fractions, as proposed in previous work [20]. The results are shown in Figure 15, where it
cannot be seen relevant effect on the behaviour of the MWD of the produced bio-oils due to interactions
between cellulose and lignin. However, it can be observed that as the concentration of K+ increases, more
light weight components are obtained in the condensed products.
Figure 15. Discretized MWD of condensed product from blends experiments.
4.1.3 Cellulose-lignin interaction effects on condensed product composition.
One of the most remarkable findings done in this work is shown in Figure 16, where the levoglucosan
yield with respect of the cellulose content in the sample is presented as function of the potassium content
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
0
100
200
300
400
500
600
700
800
900
Lignin A/L A/L 100 A/L 1000 A/L 10000
Mol
ar m
ass
(g/m
ol)
Mn
Mw
Mw/Mn
(Mw
/Mn)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
100.00 1,000.00 10,000.00
wi
Molar mass (g/mol)
Lignin A-L A-L 100 A-L 1000 A-L 10000
28
of the samples. Moreover, the results obtained in this work are compared to those obtained in a previous
work in the screen-heater with a slightly different condition (5 mbar), for only cellulose impregnated
experiments [24].
It is observed that the production of LG increased with the presence of lignin, if compared to that when
only cellulose was used; this is only valid when the concentration of K+ is low (1 and 100 ppm). When
pure components were used, for the avicel/lignin blend, a LG yield of 0.26 g/g of converted cellulose in
the sample was obtained; compared to that obtained when only cellulose was pyrolyzed (0.2 g/g) [24].
Similarly, when the concentration of potassium was 100 ppm, a LG yield of 0.17 g/g cellulose was
produced; compared to the value obtained for cellulose only (0.1 g/g) [24]. Nevertheless, when the
composition of the salts increased to 1000 and 10000 ppm (0.08 and 0.02 g/g cellulose respectively), the
LG yields remained similar to those previously obtained for cellulose only samples.
Figure 16. LG yield obtained from condensed product as a function of potassium concentration in cellulose, compared to data obtained from a previous study with only cellulose [24].
These results could indicate that there are interactions between cellulose and lignin which have an effect,
enhancing the LG yields at low potassium concentrations. It should be noted that the pressure used in
the previous work was slightly different than the used in this study (5 to 0.05 mbar, respectively), and this
factor could also have influenced the obtained results [24]. Similar results have been reported in previous
studies, as explained in Chapter 2 of this report; especially where a pyrolyzer coupled to a GC-MS was
used [32], where was also detected that the presence of lignin limited the yields of some dehydration
products from anhydrosugars, like levoglucosenone, 5-HMF, and furfural. It seems that the presence of a
liquid lignin intermediate produces a thermal ejection of LG from the reaction zone, limiting in this way its
further degradation [38]. In addition, it has also been reported that thermal degradation of LG is
suppressed in the presence of aromatic compounds [39]. This is only valid at low concentrations of
AAEMs; therefore, as has been concluded in many previous works, a pre-treatment of lignocellulose
biomass in order to remove AAEMs content as much as possible is needed.
0
0.05
0.1
0.15
0.2
0.25
0.3
1 10 100 1000 10000 100000
(g le
vogl
ucos
an /
g c
ellu
lose
)
K+ concentration (ppm)
Levoglucosan yield
Cellulose only(Marathe, 2017)
Cellulose/Lignin
29
Moreover, Figure 17 shows the glucose recovery as a function of the potassium concentration in the
blends. Glucose was obtained after hydrolysis of levoglucosan and oligomeric anhydrosugars present in
the condensed product. The glucose recovery is expressed as the ratio between the carbon in glucose
and carbon in cellulose, where also the results from a previous work are shown in order to compare the
values obtained [24]. In this case, an effect on the glucose recovery due to cellulose-lignin interactions
could not be observed while comparing to previous work results.
Figure 17. Glucose recovery after hydrolysis of the condensed product as function of potassium concentration in cellulose, compared to data obtained from a previous study with only cellulose [24].
4.2 Lignocellulose samples from poplar.
4.2.1 Cellulose-lignin interaction effects on gas, solid and condensed product yields.
Furthermore, the possible cellulose-lignin interactions different lignin content were analyzed in this part of
the study, where samples of native and treated poplar were used as feedstock. The lignin content of these
samples as presented on Table 4, which is 25%, 11.5% and 3.6% wt; as well as one untreated sample
with approximately > 10000 ppm content of salts.
The effect of the lignin content on non-condensable gas yield, for the poplar treated and untreated
samples is shown in Figure 18. It can be seen that the highest non-condensable gas yield is produced at
the untreated poplar sample, 0.03 g/g sample, which comparable with the yield obtained for the blend A/L
with 10000 ppm of potassium. This result was expected due to the high content of salt which quickly
catalyze the primary pyrolysis reactions. Unfortunately, there was not found a trend for the treated
samples with different lignin content Therefore, can be said that the content of lignin has no influence in
the gas yield for the samples used, but the AAEM content does.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1 10 100 1000 10000 100000
C g
luco
se /
C c
ellu
lose
(g/
g)
K+ concentration (ppm)
Glucose recovery from cellulose
Cellulose only(Marathe, 2017)
Cellulose/Lignin
30
Figure 18. Gas yield obtained from poplar samples experiments.
Moreover, the solid residue yields are presented in Figure 19. It can be seen that, the highest yield of
solid was obtained for the native sample (0.07 g/g sample), as expected, due to its high AAEM content;
in contrast, the lowest yield was produced from the acid leached (25% wt lignin) and the PT17 pulp
samples (3.6% wt of lignin) with 0.025 g/g sample for both. As well as the gas yield, no trend was found
due to the lignin content. Because of this, it can be said that the content of lignin, for the samples and
conditions used in this study, has no effect on the solid residue production.
Figure 19. Solid residue yield obtained from poplar samples experiments.
Figure 20 shows the condensed product yield obtained in the experiments with poplar samples. It can be
observed that, as expected, the untreated sample produced the lowest condensed product (0.68 g/g
sample); opposite to the acid leached sample (0.85 g/g sample) which generated the highest yield of
condensed product. This results reinforced fact that, it is mandatory to pre-treat the lignocellulose,
reducing the AAEM content, in order to increase the bio-oil yield.
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
Gas
yie
ld (
g/g
sam
ple)
Non-condesable gas yield
Poplar Poplar AL PT19 PT17
0.000
0.010
0.020
0.030
0.040
0.050
0.060
0.070
0.080
Sol
id y
ield
(g/
g sa
mpl
e)
Solid residue yield
Poplar PoplarAL PT19 PT17
31
No trend was found related to the lignin content of the samples, as mentioned for gas and solid yields,
therefore, an effect of the lignin content over the condensed product yield cannot be established, for the
conditions used.
Figure 20. Condensable product yield obtained from poplar samples experiments.
The mass balances of all experiments was between 78% and 89%, being for the untreated poplar the
lowest, as expected, and for the treated sample with 25% content of lignin the highest. Unfortunately, due
to the limited availability of the samples, the number of experiments performed, were three for the poplar
and AL poplar samples; and only two for PT19 and PT17. Nevertheless, as shown in Figure 21, the
reproducibility of the experiments was good.
Figure 21. Mass balance closures obtained from poplar samples experiments.
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
Con
dens
able
yie
ld (
g/g
sam
ple)
Condesable product yield (g/g sample)
Poplar Poplar AL PT19 PT17
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Clo
sure
(%
)
Mass balance closure
Poplar Poplar AL PT19 PT17
32
4.2.2 Cellulose-lignin interaction effects on molecular weight of condensed product.
The molecular numbers and the polydispersity values obtained from the condensed product of the poplar
samples are shown in Figure 22. For the treated samples (Poplar AL, PT 19, and PT17), a trend can be
highlighted, because the molecular weight numbers (Mw and Mn) decreased as the lignin content in the
sample decreased, which could indicate that the amount of oligomers decreased as the lignin content
also did. In contrast, polydispersity values increased when the lignin content was lower.
Figure 22. Molecular weight numbers and polydispersity of condensed product from poplar experiments.
4.2.3 Cellulose-lignin interaction effects on condensed product composition.
LG yields and glucose recovery were also computed for the condensed product from the poplar samples
experiments and are shown in Figure 23 and Figure 24, respectively. Interestingly, for both results, a
perfectly visible trend can be observed, LG yield and glucose recovery increased as the concentration of
lignin in the samples also did. These trends have also been reported in previous work [32], where the
concentration of LG was also higher at a 20% wt lignin content. This could also be related to the possible
thermal stabilization of anhydrosugars in the presence of aromatics previously mentioned [39].
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
0
100
200
300
400
500
600
700
Poplar Poplar AL PT 19 PT 17
Mol
ar m
ass
(g/m
ol)
Mn
Mw
Mw/Mn
(Mw
/Mn)
33
Figure 23. LG yield obtained in condensed product of poplar experiments as a function of lignin content in the feedstock.
Figure 24. Glucose recovery after hydrolysis of the condensed product as a function of lignin content in the feedstock.
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0 5 10 15 20 25 30
levo
gluc
osan
yie
ld (
g/ g
cel
lulo
se)
Lignin content (%)
Levoglucosan yield
Non treated Poplar
Treated poplar
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0 5 10 15 20 25 30
C g
luco
se/
C c
ellu
lose
(g/
g)
Lignin content (%)
Glucose recovery from cellulose
Non treated Poplar
Treated poplar
34
35
5. Possible contributions of present study towards fast pyrolysis
applications improvements
5.1 Applications of fast pyrolysis of biomass
Fast pyrolysis of lignocellulosic biomass is focused on the production of bio-oil, which can be converted
into several forms of energy carriers and into platform chemicals; representing an advantage over other
renewable technology options by providing a potential substitute of fossil feedstocks in the petrochemical
sector. The use of biomass for chemical and energy applications is part of the current EU's bioeconomy
strategy [40]. For applications of lignocellulosic biomass for energy, there are already many incentives on
the way, like electricity subsidies, tax reliefs, and bioenergy schemes [41]. For biobased chemicals, a
potential market is expected to grow quickly; however, further research and development is still needed
as well as raising awareness of policies, which are now without clear incentives for deployment [42]. In
Figure 25, an overview of the potential applications of fast pyrolysis of biomass products is presented.
Figure 25. Applications for fast pyrolysis products [43].
Fast pyrolysis of biomass has an enormous potential as a pre-treatment method to produce an effective
energy carrier, the bio-oil. One of main disadvantages of biomass, is a widely dispersed resource that has
to be harvested and transported to a conversion facility. The low bulk density of biomass, which can be
as low as 50 kg/m3, means that transport costs are high and the number of vehicle movements for
transportation to a large scale processing facility are also very high, with consequent substantial
environmental impacts [44]. Assuming that, processing is carried out in a close-by to source facility, this
conversion of biomass to a liquid by fast pyrolysis would reduce transport costs and reduce environmental
concerns as the liquid has a density of 1200 kg/m3, which represents more than ten times higher than
harvested biomass [41]. This could also reduce costs of handling by taking into account a liquid that can
be pumped. This is currently developing a concept of small decentralised fast pyrolysis plants of 50,000
Fast Pyrolysis
Gas
Char
Primary product
Processing Final
Product
Bio-oil
Separation
Conversion
Engine, turbine
Combustion
Platform chemicals
Bio-fuels
Electricity
Heat
Process heat
Process heat
36
to 250,000 tonnes per year for the production of bio-oil to be transported to a central processing plant
[44].
In addition, the direct production of high yields of liquids by fast pyrolysis have always caused attention to
focus on their use as biofuels to supplement and further replace fossil derived transport fuels. However,
the high oxygen content of bio-oil and its non-miscibility or incompatibility with hydrocarbon fuels has
made this possible transition a very difficult process [45]. Because the qualities above mentioned, bio-oil
needs to be post-treated, commonly known as upgrading, in order to be able for using it as fuel. Among
the several methods that have been proposed for the upgrading of bio-oil, hydrodeoxygenation can be
highlighted. Practically, it is a process of liquid bio-oil that rejects oxygen as water by catalytic reaction
with hydrogen. This process gives a naphtha-like product that requires orthodox refining to derive
conventional transport fuels. Therefore, it could to take place in a conventional refinery to take advantage
of know-how, existing processes and economies of scale. It has been projected that a typical yield of
naphtha equivalent from biomass could be about 20% by weight or 55% in energy terms, excluding
provision of hydrogen [46].
On the other hand, some platform chemicals could also be obtained from pyrolysis bio-oil after some
separation or conversion methods. Although bio-oil contains in excess of 1000 individual chemicals, few
are present in sufficient concentrations to be feasible its recovery. Among these compounds,
levoglucosan has a great potential of being recovered in high purity and high yield, nevertheless until
recently has been perceived as having an upcoming market value; because of more attention has been
paid to its potential of producing sugars by hydrolysis or fermentation [47].
Moreover, under the biorefinery concept development, fast pyrolysis has the potential of both producing
bio-oil as a precursor for second generation biofuels for transport, and generating platform chemicals. A
key feature of the biorefinery concept is the co-production of fuels, chemicals and energy. There is a clear
economic advantage in building a similar flexibility into the biofuels market by devoting part of the biomass
production to the manufacture of chemicals [43]. In fact, under this concept, a pilot plant has been recently
developed by BTG company, in The Netherlands. It is an innovative two-step conversion process were a
fast pyrolysis of biomass is followed by a fast pyrolysis of the previously produced bio-oil. Three main
products are then obtained: pyrolytic lignin, which is suitable for the production of renewable bitumen or
various resins, as a replacement for fossil phenol; pyrolytic sugars, suitable for the production of furan-
based resins and can be obtained with various water contents; and extractives, a raw material for specialty
chemicals [48].
5.2 Reinforcement of fast pyrolysis current behaviour models
As mentioned on Chapter 2.1 in this study, biomass feedstock comes in many varieties, but have common
main constituents as cellulose, hemicellulose, and lignin. As the relative proportions of these constituents
vary depending on many factors, customization of the pyrolysis process conditions is required to produce
a desired product profile, i.e. pyrolytic sugars. Therefore, recognizing these sources of variation,
mathematical models could be built to further applications on simulators, allowing to set reactor settings
in order to achieve an optimal operation. These considerations might include biomass classification, feed
rate, moisture content, particle size, and thermal and mass transport gradients of the desired reactor [49].
The recent advances in the understanding of the fundamental reaction pathways have been widely
described, including the description of pyrolysis as a two-step process, primary and secondary pyrolysis
37
reactions, the effect of the operational pressure, the effect of the presence of an intermediate liquid
compound, and the influence of inorganic salts. Among the several approaches that have been used to
model, the description of biomass pyrolysis as the sum of the contributions of its individual components
has been highlighted. This method has been widely employed to examine the reactions pathways,
confirming that pyrolysis of biomass appeared to behave as the sum of its constituents [50].
Nevertheless, evidence of the interactions among the components under pyrolytic conditions would give
a conceptual guide to modify the understanding of pyrolytic mechanism of biomass from this point of
component view, and help to design a new-concept of co-pyrolysis process for promoting the production
of value-added compounds. The interactions between chemical components in biomass under pyrolytic
conditions have been evidenced and discussed within recent years, as mentioned in Chapter 2.6.
However, the understanding has been greatly limited and two main issues were needed to be addressed
for specifying the possible interaction mechanisms: sample preparation representing the natural
information between components in biomass, chemical pathways for the interaction and quantitative
assessment for the interactive extent [16].
In this regard, the present study intended to solve the two above mentioned issues, by using the already
described experimental configuration and a quantitative estimation of the product composition. Therefore,
the results obtained could provide valuable experimental information which can be used to create a better
and more complete model for the prediction of the product selectivity. However, experimental studies still
have to be done to enhance the modelling of pyrolysis process towards building of a robust simulation
technology tool, and finally help this promising technology to an industrial application.
38
39
6. Conclusions
In this study, the possible interactions between cellulose and lignin during the primary reactions of fast
pyrolysis of different feedstocks were analyzed. The experiments were carried out in a reactor developed
in SPT group, a screen-heater which enables a very fast heating rate (5000° C/s), short vapor residence
time (< 79 ms), and fast quenching of the products; in order to recover the primary products of the reaction.
The obtained results from the model mixtures were compared to those previously reported using also the
screen-heater, with similar reaction conditions. It was not found a considerable effect, for the non-
condensable gas, solid residue, condensed product yields, due to interactions between cellulose-lignin.
In addition, the glucose recovery from cellulose was also computed in order to estimate the total sugar
content from anhydrosugars; discarding also relevant effects due to cellulose-lignin interaction.
However, it should be highlighted from this work, that interactions between cellulose and lignin were found
when comparing the obtained levoglucosan yields from the condensed product of the blends with the
results previously reported for only cellulose experiments. The presence of lignin enhances the production
of LG, when the potassium concentrations remain low (till 100 ppm). This could be attributed mainly to
two processes: 1) lignin promotes the thermal ejection of the cellulose derivatives from the reaction zone,
and 2) thermal degradation of levoglucosan is partially suppressed by the presence of aromatic
compounds. Moreover, results obtained from poplar samples also did not show a consistent to relate the
lignin content by analyzing the non-condensable gas, solid residue and condensed product yields.
Nevertheless, a trend was obtained when comparing the LG yields and glucose recovery from the
condensed product composition. It seems that both yields increase as the lignin content present in the
sample increases, which could be attributed as an effect of cellulose-lignin interactions in the pyrolysis
process.
6.1 Recommendations
In order to reinforce the results obtained in this work, using similar conditions and feedstock for the screen-
heater, two recommendations can be done for analyzing the composition of the condensed product: 1) to
analyze the composition of anhydrosugars with a higher degree of polymerization (cellobiosan,
cellotriosan, cellotetrasan); 2) to analyze the composition of dehydration products from anhydrosugars
(levoglucosenone). These could help for the previously developed models to be more robust, in order to
be able to predict more accurately lignocellulosic biomass behaviour during fast pyrolysis.
In addition, the screen-heater could also be used to study the effects of possible catalysts under the
primary reactions of fast pyrolysis, using firstly model mixtures as the ones used in this work, to further
analyze its effect on native biomass samples. This experimental set-up could be used to help building the
basis for the development of biomass fast pyrolysis towards its many applications as a possible
replacement of fossil resources.
40
41
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45
Acknowledgement
Firstly, I would like to thank Pilar for helping me to choose a master thesis assignment and for the
opportunity to do this research at the SPT group, also for her supervision during the past few months and
for always being available and willing to help me. Secondly, I would like to thank Sascha also for giving
me the opportunity to be part of this amazing research group, and for his valuable feedback. I also would
like to thank Marteen for always being available to help me develop the socio-economical part of this
work.
Special thanks should be given to all the technical staff of the SPT group, Erna and Benno for all the help
in the process. I would also like to thank Thimo for all the help on the laboratory and also for providing me
with the poplar samples. Pushkar, thanks for all your advises, really help me a lot during my work.
